General Chemistry Principles, Patterns, and Applications, 2011

structures; the accompanying release of heat increases the entropy of the surrounding
environment so that Suniv > 0.
Note the Pattern
Any organism in equilibrium with its environment is dead.
Extracting Energy from the Environment
Although organisms employ a wide range of specific strategies to obtain the energy they need to live and
reproduce, they can generally be divided into two categories: organisms are either phototrophs (from the
Greek photos, meaning “light,” and trophos, meaning “feeder”), whose energy source is sunlight,
or chemotrophs, whose energy source is chemical compounds, usually obtained by consuming or breaking
down other organisms. Phototrophs, such as plants, algae, and photosynthetic bacteria, use the radiant
energy of the sun directly, converting water and carbon dioxide to energy-rich organic compounds,
whereas chemotrophs, such as animals, fungi, and many nonphotosynthetic bacteria, obtain energy-rich
organic compounds from their environment. Regardless of the nature of their energy and carbon sources,
all organisms use oxidation–reduction, or redox, reactions to drive the synthesis of complex biomolecules.
Organisms that can use only O2 as the oxidant (a group that includes most animals) are aerobic
organisms that cannot survive in the absence of O2. Many organisms that use other oxidants (such as
SO4 2− , NO3 − , or CO3 2− ) or oxidized organic compounds can live only in the absence of O2, which is a deadly
poison for them; such species are called anaerobic organisms.
The fundamental reaction by which all green plants and algae obtain energy from sunlight
is photosynthesis, the photochemical reduction of CO2 to a carbon compound such as glucose.
Concurrently, oxygen in water is oxidized to O2 (recall thathν is energy from light):
Equation 18.49
6CO2 + 6H2O ® hvC6H12O6photosynthesis + 6O2
This reaction is not a spontaneous process as written, so energy from sunlight is used to drive the
reaction. Photosynthesis is critical to life on Earth; it produces all the oxygen in our atmosphere.
In many ways, chemotrophs are more diverse than phototrophs because the nature of both the reductant
(the nutrient) and the oxidant can vary. The most familiar chemotrophic strategy uses compounds such as
glucose as the reductant and molecular oxygen as the oxidant in a process called respiration. (For more
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